Electric vehicles

Granny charger vs wallbox: miles per night and cost on a 6-hour off-peak window

Last checked: 15 October 2026 · PlugLater

A granny charger is the portable lead that plugs into an ordinary 13 A socket. A wallbox is a charger fixed to the wall on its own circuit. The electricity costs the same per kWh through either one. What changes is how many kWh you can get into the car during the cheap hours, and whether that covers your week.

The power gap

3-pin granny lead: about 1.8 to 2.3 kW. UK granny leads are limited to 10 A (2.3 kW) on a normal socket, and many can be set lower, to 8 A (about 1.8 kW). EV-Database uses 2.3 kW for its wall-plug charging times.
Single-phase wallbox: up to 7.4 kW. That is 32 A on a normal home supply. EV-Database notes most UK homes do not have the three-phase supply needed for 11 kW.
The car can cap it. Every car in the table below has an 11 kW on-board AC charger according to EV-Database, so none of them is held back at 7.4 kW. Check your own car’s figure on its EV-Database page.

Miles added in one 23:30–05:30 window

Six hours × charger kW = kWh delivered. kWh × the car’s miles per kWh = miles added. So 1.8 kW gives 10.8 kWh, 2.3 kW gives 13.8 kWh and 7.4 kW gives 44.4 kWh. Charging losses are not included, and losses are usually a bigger share at low power, so treat the granny columns as the most optimistic case.

Miles added in six hours, before losses
Car (EV-Database, real range)	mi/kWh	1.8 kW	2.3 kW	7.4 kW
Tesla Model 3 RWD	4.59	50	63	204
Vauxhall Corsa Electric 54 kWh	4.03	44	56	179
Tesla Model Y RWD	3.92	42	54	174
Nissan Leaf Standard Range 52 kWh	3.77	41	52	167
MG4 Urban Comfort Standard Range	3.70	40	51	155 (full battery)
Kia EV3 Long Range	3.58	39	49	159
BYD Atto 3 Evo RWD Design	3.34	36	46	148

A car with a small battery cannot take all of a 7.4 kW window. The MG4 Standard Range has 41.9 kWh usable, less than the 44.4 kWh a 7.4 kW charger can deliver in six hours, so its limit is a full battery: about 155 miles from empty. The miles per kWh figures are the same ones used in EV charging costs by car, which covers all 142 cars.

Related: Renault 5 and UK EV tariffs compared.

Worked example: what one night costs

1.8 kW for six hours: 10.8 kWh × 7.6p = 82p. At 26.32p it would be £2.84.
2.3 kW for six hours: 13.8 kWh × 7.6p = £1.05. At 26.32p it would be £3.63.
7.4 kW for six hours: 44.4 kWh × 7.6p = £3.37. At 26.32p it would be £11.69.

Per kWh, the granny lead and the wallbox cost the same. The wallbox costs more per night only because it puts more energy in.

Does your week fit in the cheap hours?

Weekly miles ÷ miles per kWh = kWh you need each week. Compare that with 7 nights × the kWh your charger adds in six hours. Anything left over is bought outside the window, at your day rate.

Example 1: Vauxhall Corsa Electric, 8,000 miles a year. That is about 154 miles a week, or 38.2 kWh at 4.03 mi/kWh. A 2.3 kW granny lead adds 13.8 kWh a night, so 2.8 nights a week covers it. On EV-Database’s cold-weather combined figure (299 Wh a mile, 3.34 mi/kWh) it is 46.1 kWh, still about 3.3 nights. For this driver the cheap window is not the problem. The questions are whether the socket is suitable and whether the tariff works with the setup (both below).

Example 2: Kia EV3 Long Range, 20,000 miles a year. That is about 385 miles a week, or 107.4 kWh at 3.58 mi/kWh. Seven nights on a 2.3 kW granny lead add 96.6 kWh, which leaves 10.8 kWh a week to buy outside the window. At 26.32p that is £2.85 a week, against 82p if it fitted at 7.6p. In cold weather (332 Wh a mile) the shortfall grows to about 31.2 kWh a week. A 7.4 kW wallbox adds 44.4 kWh a night, so the same week fits in about 2.4 nights.

Both examples assume the car is plugged in every night and every mile is charged at home. Plug in your own mileage in the EV calculator.

Smart tariffs: check compatibility before you buy

Intelligent Octopus Go works by connecting to a compatible car or a supported smart charger through the Octopus app. A plain 3-pin lead is not a charger Octopus can control, so whether you can join with one depends on whether Octopus supports your car directly. Octopus publishes a checker on its Intelligent Octopus Go page. Check your exact car and setup there before you switch, and don’t assume a granny lead will be accepted.

The site’s Intelligent Octopus Go guide covers how the tariff’s cheap hours work.

Safety with a 3-pin lead

Plug straight into a wall socket. Never use an extension lead, cable reel or adaptor.
Use a socket in good condition, on an RCD-protected circuit. Older wiring may not cope with hours of continuous load, so have an electrician check it before you rely on it.
Leave the lead on 10 A or lower on an ordinary socket. Higher settings are meant for EV-rated sockets on their own circuit.
Follow your car maker’s guidance for its lead. The Electrical Safety Roundtable describes granny chargers as being for occasional or emergency use, not as a main charging solution.

What a wallbox costs

Two published starting prices, checked on 15 October 2026:

Octopus Energy: smart chargers from £949 including standard installation.
Ohme: ePod from £949 and Home Pro from £999, including standard installation.

“Standard installation” has limits that vary by installer: cable length, where the consumer unit is and whether it needs upgrading. Get a survey and a written quote. This page doesn’t give a payback period, because that needs your quote. To work one out: payback in years = your installed price ÷ (kWh a year you would otherwise buy outside the window × (your day rate − your night rate)).

A wallbox does not create a cheap rate. The tariff does.

Questions people ask

Is a granny charger cheaper per kWh than a wallbox?

No. A kWh costs the same through either one. On 7.6p it is 7.6p whichever lead it goes through. The difference is how many kWh fit into the six cheap hours: about 10.8 to 13.8 kWh from a 3-pin lead, against 44.4 kWh from a 7.4 kW wallbox.

How many miles does a granny charger add overnight?

In six hours at 2.3 kW it adds 13.8 kWh. That is about 46 miles in a BYD Atto 3 at 3.34 mi/kWh, and about 63 in a Tesla Model 3 RWD at 4.59 mi/kWh, before losses. At 1.8 kW it is about a fifth less.

Can I use a granny charger on Intelligent Octopus Go?

It depends on whether Octopus can connect to your car directly, because it cannot control a plain 3-pin lead. Check your car and setup on Octopus’s Intelligent Octopus Go page before switching.

Is it safe to use a granny charger every night?

Safety bodies describe granny chargers as being for occasional or emergency use. If you rely on one, plug it straight into a sound wall socket on an RCD-protected circuit, never use an extension lead, and have an electrician check the wiring.

How much does a wallbox cost to install?

Octopus Energy and Ohme both publish starting prices of £949 including standard installation (checked 15 October 2026), and Ohme’s Home Pro is from £999. What you actually pay depends on your survey.

Is this financial advice?

No. These are worked examples from published rates and EV-Database estimates. Check your own tariff, quotes and wiring before you decide.

Not financial advice. These are worked examples from published rates and EV-Database consumption estimates, for planning only. Your own tariff, day rate, standing charge, car, driving and weather will change the numbers. Check your bill and your supplier’s current terms before you switch or buy anything.

Buying a wallbox: installation cost compared, Pod Point vs Ohme and the OZEV grant checker.

Sources

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